The present disclosure relates generally to automobile steering systems and, more particularly, to a method and apparatus for motor velocity measurement compensation in the damping of electric power assist steering systems.
In a vehicle equipped with electric power steering (EPS), a steering assist torque may be provided by an electric motor coupled to a steering column or shaft. Generally speaking, in such systems, a controller derives an assist torque signal, along with a xe2x80x9creturn to centerxe2x80x9d (i.e., neutral position) torque signal, and thereafter sums these torque signals to produce a motor command signal. The first of these torque signals provides the power steering boost torque, and the latter provides return to center bias torque. However, an EPS system further has a free rotational oscillation resonance associated therewith that, if left undamped, may not result in a crisp, controlled feel to the steering.
Accordingly, input-dependent active damping features have been provided with EPS systems to aid in the free control response of a vehicle. An example of such an active damping system is disclosed in U.S. Pat. No. 5,919,241 (the ""241 patent), assigned to the assignee of the present application, and the contents of which are incorporated herein by reference. In the ""241 patent, the active damping provides an active damping torque signal that is further summed along with the assist torque signal and the return to center torque signal to produce the motor command signal. In turn, the active damping torque signal is derived as a function of a filtered steering shaft position and a sensed vehicle velocity. A filtering means for generating the filtered steering shaft position includes amplitude and phase characteristics of a differentiator from 0 Hz through the resonant frequency of free rotational oscillation. At rotational oscillation frequencies greater than the resonant frequency of free rotational oscillation, the amplitude and phase lead characteristics are decreasing, relative to those of a differentiator.
Although subsequent improvements have been developed for active damping systems to further enhance system stability without compromising on-center feel, one particular shortcoming has been discovered in those systems wherein a motor velocity measurement is obtained by differentiating motor position. Because a differentiating position approach is an inherently numerically noisy approach, a velocity measurement disturbance results in a tactile torque disturbance in the handwheel, in addition to an audible disturbance.
The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a method for generating a compensated motor velocity output value for an electric power steering motor. In an exemplary embodiment, the method includes determining a first motor velocity value and a second motor velocity value. A measured vehicle speed is then compared to a determined vehicle speed range. If the measured vehicle speed exceeds the determined vehicle speed range, then the compensated motor velocity output value is set to the first motor velocity value, and if the measured vehicle speed is less than the determined vehicle speed range, then the compensated motor velocity output value is set to the second motor velocity value. However, if the measured vehicle speed is within the determined vehicle speed range, then the compensated motor velocity output value is a linearly blended value of the first motor velocity value and the second motor velocity value.
In a preferred embodiment, the first motor velocity value is determined by using a first number of motor positions in conjunction with a motor position signal, the second motor velocity value is determined by using a second number of motor positions in conjunction with the motor position signal. Preferably, the first number of motor positions is less than the second number of motor positions.